A small voltmeter using a arduino nano.

A very simple project for the electronic part, a arduino nano and two resistance 100kOhms and 10kOhms. Flow the sketch to connect everything. 

The software can detect is a battery is good enough using some setpoint. You can change the setpoint using the pushbutton. The setpoint is saved in the EEPROM of the arduino, you do not need to set up again after switch off the voltmeter.